Rename impl_standard_builder from impl_default_via_builder

I have Plans for this macro.  In particular:

 * I have a wip branch which tests that the Builder can be
   deserialised from an empty config (ie, that config reading
   of a config with a blank section for this item works).

 * I think we should autogenerate $Config::builder(),
   and promote that, rather than $ConfigBuilder::default().
   This macro could do that.